A speed ratio of a transmission is changed by altering a torque transmission route and a torque transmitting point. In the transmission in which a frictional engagement device is arranged on the torque transmission route, the torque transmitting capacity of the transmission is governed by an engagement force. One example of the conventional transmission in which the speed ratio and the torque transmitting capacity are thus controlled is described in Japanese Patent Laid-Open No. 2011-163508.
Specifically, Japanese Patent Laid-Open No. 2011-163508 describes a belt-driven continuously variable transmission comprising a drive pulley (i.e., a primary pulley), a driven pulley (i.e., a secondary pulley) and a belt running between those pulleys. Each pulley comprises a fixed sheave and a movable sheave that reciprocate with respect to the fixed sheave to change a width of a belt groove formed therebetween. Each primary pulley and the secondary pulley is individually provided with a hydraulic chamber for establishing a thrust force applied to the movable sheave. Each hydraulic chamber is connected to a feeding valve for delivering oil thereto from a hydraulic source, and to a discharging valve for draining the oil to a drain site. A groove width of one of the pulleys (i.e., the primary pulley) is narrowed to increase a running diameter of the belt to cause an upshifting by opening the feeding valve connected thereto to deliver the oil thereto. By contrast, the groove width of the primary pulley is widened to decrease the running diameter of the belt to cause a downshifting by opening the discharging valve connected thereto to discharge the oil therefrom. On the other hand, a belt clamping pressure of the other pulley (i.e., the secondary pulley) is increased to increase a torque transmitting capacity by opening the feeding valve connected thereto to deliver the oil thereto. By contrast, the belt clamping pressure of the secondary pulley is reduced to reduce the torque transmitting capacity by opening the discharging valve connected thereto to discharge the oil therefrom.
According to the teachings of Japanese Patent Laid-Open No. 2011-163508, a balance piston solenoid valve is used as the feeding valve and the discharging valve. In the solenoid valve, a piston integrated with a needle-shaped or shaft-shaped valve element is held in a cylinder while being allowed to reciprocate in an axial direction. An inlet port and an outlet port are formed in the chamber holding the piston, and the inlet port is connected to a high pressure site and the outlet port is connected to a low pressure site. The solenoid valve is closed by pushing the valve element onto an opening end of the hydraulic chamber side of the outlet port. The hydraulic chamber is connected to an opposite chamber (as will be tentatively called the “control chamber”) across the piston through a communication passage on which an orifice is formed. The control chamber is also connected to the low pressure site and a solenoid is arranged in the control chamber to selectively provide a communication between the control chamber and the low pressure site. Specifically, pressure in the control chamber is lowered by opening the solenoid so that the piston is withdrawn from a valve seat toward the control chamber to open the valve. By contrast, pressure in the control chamber is raised by closing the solenoid so that the piston is pushed onto the valve seat to close the outlet port thereby closing the valve.
The feeding valve and the discharging valve taught by Japanese Patent Laid-Open No. 2011-163508 is opened by applying a current thereto, and an opening degree thereof is changed according to the current value. However, those valves do not have a function to control a pressure, and hence a speed ratio and a belt clamping pressure (i.e., a torque transmitting capacity) are controlled by controlling the feeding valve and the discharging valve by a feedback method. To this end, for example, a pressure difference in the hydraulic chamber of the drive pulley is calculated based on a difference between a target speed ratio and an actual speed ratio, and a control amount of the feeding valve or the discharging valve is calculated based on the pressure difference and a control gain. Likewise, in order to control pressure in the secondary pulley, a control amount of the feeding valve or the discharging valve is calculated based on a pressure difference between a target pressure determined based on a drive demand such as an accelerator opening and an actual pressure, and the feeding valve or the discharging valve is controlled based on the control amount thus calculated.
In general, a speed ratio of the belt-driven continuously variable transmission is calculates as a ratio between rotational speeds of the drive pulley and the drivel pulley, however, the rotational speeds of those pulleys are slightly changed during propulsion of the vehicle by a various kinds of factors. Likewise, the pressure in the hydraulic chamber of the secondary pulley, that is, the belt clamping pressure is also changed slightly by changes in the rotational speed, an initial pressure and so on. This means that the oil is always delivered or discharged to/from the hydraulic chamber of each pulley. For example, if the pressure in the hydraulic chamber of the primary pulley is changed, a clearance between the fixed sheave and the movable sheave thereof, that is, the running diameter of the belt is changed. Consequently, in the secondary pulley, a groove width is widened or narrowed in response to a change in the running diameter of the belt running therein. That is, pressure in the hydraulic chamber of the secondary pulley is raised or lowered by an axial movement of the movable sheave. As a result, the feeding valve or the discharging valve connected to the hydraulic chamber is opened or closed to adjust the pressure in the hydraulic chamber to the target pressure.
Thus, in the belt-driven continuously variable transmission, pressure change in the hydraulic chamber of the drive pulley exerts influence to the pressure in the hydraulic chamber of the secondary pulley. Likewise, when the pressure in the hydraulic chamber of the secondary pulley is increased or lowered to control the belt clamping pressure, the pressure in the hydraulic chamber of the drive pulley is changed by such pressure change. That is, if the hydraulic pressure is changed not only to control the speed ratio but also to control the belt clamping pressure simultaneously, the hydraulic pressure may be changed excessively to cause a pressure hunting. Such disadvantage may also be caused in the system in which the feeding valve and the discharging valve are not the above-explained balance piston valve. FIG. 21 shows a situation in which the pressure in the hydraulic chamber of the secondary pulley is changed significantly by executing an upshifting under conditions that a downshifting is slightly in execution while maintaining the pressure in the hydraulic chamber of the secondary pulley to a constant level to maintain the belt clamping pressure to a constant level. During the upshifting, the pressure in the hydraulic chamber is increased to widen a groove width in the primary pulley and hence the groove width in the secondary pulley is widened compulsory. Consequently, the pressure in the hydraulic chamber of the secondary pulley may be raised by a reduction in the capacity thereof, and in addition, the oil may be delivered thereto to control the belt clamping pressure. For these reasons, as shown in FIG. 21, the pressure in the hydraulic chamber of the secondary pulley may be fluctuated significantly to cause the hunting after commencement of the upshifting.